In order to evaluate the role of LCAT in HDL metabolism, a 6.2 kb fragment of the human LCAT (hLCAT) gene was utilized to develop three separate transgenic mouse lines overexpressing hLCAT at plasma levels 10,14 and 100 fold higher than control (1 ug/ml) mice. LCAT activity in 45 heterozygous and homozygous mice ranged from 582plus/minus 92 to 3695q340 nmol/ml/h (NL=31q4 nmol/ml/h). Northern blot hybridization analysis demonstrated tissue specific expression of hLCAT in mouse liver. Compared to 24 age and sex-matched siblings, transgenic mice had elevated plasma TC (133-237% of NL), CE (141-267% of NL) and HDL-C (123-209% of NL) but similar plasma levels of triglycerides, PL, B-containing lipoproteins, apoA-I and apoA-II. FPLC analysis of hLCAT TG plasma revealed larger sized HDL particles enriched in CE and PL. Age/sex matched transgenic (M=7, F=8) and control (M=14, F=13) animals were then placed on a high chol-fat (HF) diet for 21 d to investigate the potential role of LCAT in modulating dietary responses. Pre-diet lipid values (mg/dl) in controls were TC=97plus/minus11, HDL=72q10 and in transgenics were TC=121 plus/minus16, HDL-C=84 plus/minus17. Post-diet lipid values in controls were TC=290plus/minus55, HDL= 85plus/minus15, and in transgenics were TC=313plus/minus83, HDL=115plus/minus27. Thus, on the HF diet transgenic mice had significantly higher (pless than0.05) HDL-C as well as reduced TC/HDL ratios than controls, without differences in Tg, PL, CE, LCAT mass and activity. FPLC analysis of transgenic mouse plasma revealed significant increases in HDL-C, CE and PL with reciprocal decreases in IDL/LDL-C, CE and PL. Mouse 125I-apoA-I and 131I-apoA-II kinetic studies were performed to investigate the underlying mechanisms by which LCAT overexpression leads to increased HDL levels. ApoA-I and apoA-II catabolism was delayed in transgenic (N=5) (FCR=1.9 and 3.4d-1) when compared to control (FCR=2.6 and 4.2d-1) mice. Thus, hyperalphalipoproteinemia in transgenic mice overexpressing hLCAT results from delayed catabolism of HDL.